Imaging method and device employing sherar waves

ABSTRACT

An imaging method for observing the propagation of a shear wave simultaneously at a multitude of points in a diffusing viscoelastic medium. The shear wave is caused to be generated by firing at least one focused ultrasound compression wave into the viscoelastic medium by means of an array of transducers, and then emitting at a fast rate and using the same array of transducers, unfocused ultrasound compression waves serving to obtain a succession of images of the medium, and processing the images obtained in this way in deferred time in order to determine the movements of the medium during the propagation of the shear wave.

The present invention relates to imaging methods and apparatuses usingshear waves.

More particularly, the invention relates to an imaging method usingshear waves for observing a diffusing viscoelastic medium containingparticles that reflect ultrasound compression waves, said methodcomprising:

-   -   a) an excitation step during which an elastic shear wave is        generated in the viscoelastic medium;    -   b) an observation step during which the propagation of the shear        wave is observed simultaneously at a multitude of points in an        observation field in the viscoelastic medium, this observation        step comprising the following substeps:        -   b1) causing an array of transducers that are controlled            independently of one another to emit into the viscoelastic            medium a succession of unfocused ultrasound compression wave            shots at a rate of at least 500 shots per second; and        -   b2) causing sound signals received from the viscoelastic            medium to be detected and recorded in real time, said            signals comprising the echoes generated by the unfocused            ultrasound compression wave interacting with the reflecting            particles in said viscoelastic medium; and    -   c) at least one processing step during which:        -   c1) the sound signals received successively from the            viscoelastic medium during substep b2) are processed in            order to determine successive propagation images of the            shear wave; and        -   c2) at least one movement parameter of the viscoelastic            medium is determined at different points of the observation            field.

This produces a “motion picture” clearly illustrating the propagation ofthe shear wave through the viscoelastic medium, which can make itpossible to perform qualitative and/or quantitative analysis in order toidentify zones having hardness that differs from the hardness of theremainder of the viscoelastic medium, or zones having relaxation timethat differs from the relaxation time of the remainder of theviscoelastic medium.

Document WO-A-00/55616 describes an example of such a method, in whichshear waves are generated at the surface of the viscoelastic medium.That method gives full satisfaction in particular when imaging zonessituated relatively close to the surface of the viscoelastic medium.However that known method does not enable certain zones to be observedin the viscoelastic medium, and in particular:

-   -   zones that are sufficiently deep to be unreachable by shear        waves generated at the surface (shear waves attenuate quickly);        and    -   shadow zones that are masked by obstacles (in particular        portions of a patient's skeleton or liquid zones such as liquid        cysts) which impede the propagation of shear waves.

In addition, if the observation field is partially in a shadow zone, itcan be necessary to move the shear wave generator device during anobservation, which is tedious for the user.

Finally, the shear wave generator device is relatively heavy andcomplicates the apparatus.

A particular object of the present invention is to mitigate thosedrawbacks.

To this end, according to the invention, a method of the kind inquestion is characterized in that during excitation step a) the elasticshear wave is caused to be generated by causing at least one focusedultrasound wave to be emitted into the viscoelastic medium by said arrayof transducers, the focusing and the timing of said focused ultrasoundwave, and the timing of said unfocused ultrasound wave being adapted sothat at least some of said unfocused ultrasound waves penetrate into theobservation field while the shear wave is propagating in the observationfield.

Thus, the same array of transducers can be used both for generating theelastic shear wave in selected manner in the observation field, and forsubsequently observing said propagation by virtue of the fact that theimaging apparatus is adapted to generate either focused ultrasound wavesenabling the elastic shear waves to be generated or unfocused ultrasoundwaves enabling the propagation of the shear wave to be observed, andsuitably selecting:

-   -   the timing of the various emissions; and    -   the point(s) on which the focused ultrasound wave is/are        focused.

The imaging method of the invention is thus easy to implement for a userusing apparatus that is relatively simple and lightweight. The inventionis thus of very low cost compared with competing techniques such asmagnetic resonance imaging (MRI), and where appropriate it makes itpossible to establish outpatient imaging systems that can be used forpreoperative imaging, postoperative imaging, and even for imaging whilean operation is in progress.

By way of example, in medical applications, the method of the inventioncan make it possible for cancerous zones within the tissues of a patientto be identified effectively. Shear waves propagate through cancerouszones in very different manner than through adjacent zones. Thisidentification can be performed much more easily than by conventionalobservation using simple ultrasound echography, since the propagation ofshear waves is a function of the shear modulus of the medium, which isitself highly variable between zones of healthy tissue and a zone ofcancerous tissue: typically, the shear modulus varies over a ratio of 1to 30 between a healthy zone and a cancerous zone, whereas thecompression modulus, which governs the propagation of compressionsoundwaves as used in ultrasound echography varies only by about 5%between healthy tissue and cancerous tissue.

Similarly, it is thus possible to identify zones of necrosis withintissue, for example tumor zones that have been subjected to ultrasoundhyperthermia treatment, in particular for the purpose of evaluating theeffectiveness of the hyperthermia treatment.

Another possible application of the invention relates to quantitativelyevaluating the degree of fibrosis of the liver, which is an importantparameter in liver disease, in particular hepatitis C.

It should be observed that the invention makes it possible to generatethe shear wave, and to observe its propagation, including through a zoneof liquid or through a bone barrier (skull, rib cage, etc.) completelyor partially masking the observation field, since it is possible tofocus ultrasound waves through such barriers (see in particular documentWO-A-02/32316 or French patent application No. 02/10682 of Aug. 28,2002).

In preferred implementations of the method of the invention, it isoptionally possible also to have recourse to one or more of thefollowing dispositions:

-   -   during substep b2), in order to determine said movement        parameter, a plurality of successive propagation images (e.g. by        correlation, Doppler, etc.) are compared with a common reference        image of the viscoelastic medium, the reference image being        determined by firing at least one unfocused ultrasound        compression wave into said viscoelastic medium and then        detecting and recording echoes generated by said unfocused        ultrasound compression wave on interacting with the reflecting        particles in the viscoelastic medium (thus improving the        accuracy with which the movement parameter is measured (e.g.        displacement) in the viscoelastic medium, in particular for        movement of small amplitude (typically less than 30 micrometers        (μm) with the soundwave excitation technique used herein));    -   step a) is preceded by an initial observation step a0) during        which at least one unfocused ultrasound compression wave is        fired and then echoes generated by said unfocused ultrasound        compression wave interacting with the reflecting particles in        the viscoelastic medium are detected and recorded, said echoes        corresponding (directly or indirectly) to an initial image of        the viscoelastic medium, and during substep b2), said initial        image constitutes said reference image for processing at least        some of the successive displacement images;    -   during initial observation step a0), a plurality of unfocused        ultrasound compression waves are fired in succession and then        echoes generated by each unfocused ultrasound compression wave        interacting with the reflecting particles of the viscoelastic        medium are detected and recorded, said echoes corresponding        (directly or indirectly) to a plurality of successive images of        the viscoelastic medium, and said initial image of the        viscoelastic medium is determined by combining said successive        images;    -   said movement parameter is a displacement of the viscoelastic        medium;    -   the focused ultrasound wave emitted during excitation step a)        presents a frequency f lying in the range 0.5 megahertz (MHz) to        15 MHz, and is emitted for a duration of k/f seconds, where k is        an integer lying in the range 50 to 5000 and f is expressed in        hertz (Hz);    -   the focused ultrasound wave emitted during excitation step a)        presents a frequency lying in the range 0.5 MHz to 15 MHz and is        emitted during a succession of emission periods separated by        rest periods, the emission periods following one another at a        rate lying in the range 10 to 1000 emissions per second;    -   the focused ultrasound wave emitted during excitation step a) is        a linear combination (in particular a sum) of two monochromatic        signals having respective frequencies f1 and f2 such that 20        Hz≦|f1−f2|≦1000 Hz;    -   the focused ultrasound wave emitted during excitation step a) is        focused simultaneously on a plurality of points;    -   image processing step c) is followed (immediately or otherwise)        by a mapping step d) during which, on the basis of variation in        the movement parameter over time, at least one shear wave        propagation parameter is calculated at at least some points of        the observation field in order to determine a map of said        propagation parameter in the observation field;    -   the shear wave propagation parameter which is calculated during        mapping step d) is selected from shear. wave speed, shear        modulus, Young's modulus, shear wave attenuation, shear        elasticity, shear viscosity, and mechanical relaxation time; and    -   steps a) to d) are repeated successively while emitting        different focused ultrasound waves during successive excitation        step a), and then combining the maps obtained during the        successive mapping step d) in order to calculate a combination        map of the observation field.

Furthermore, the invention also provides an imaging apparatus forimplementing a method according to any preceding claim using shear wavesto observe a diffusing viscoelastic medium containing particles thatreflect ultrasound compression waves, the apparatus comprising an arrayof transducers that are controlled independently of one another by atleast one electronic central unit adapted:

-   -   to cause at least one elastic shear wave to be generated in the        viscoelastic medium;    -   to observe the propagation of the shear wave simultaneously at a        multitude of points in an observation field in the viscoelastic        medium by causing said array of transducers to emit into the        viscoelastic medium a succession of unfocused ultrasound        compression wave shots at a rate of at least 500 shots per        second, then causing said array of transducers to detect in real        time and record in real time sound signals received from the        viscoelastic medium, the sound signals comprising the echoes        generated by the unfocused ultrasound compression wave        interacting with the reflecting particles of said viscoelastic        medium; and    -   processing the successive sound signals received from the        viscoelastic medium to determine successive propagation images        of the shear wave, and then determining at least one movement        parameter of the viscoelastic medium at different points of the        observation field; the apparatus being characterized in that the        electronic central unit is adapted to cause the elastic shear        wave to be generated by causing at least one focused ultrasound        wave to be emitted into the viscoelastic medium by said array of        transducers, the focusing and the timing of said focused        ultrasound wave, and the timing of said unfocused ultrasound        wave being adapted so that said unfocused ultrasound waves reach        the observation field during the propagation of the shear wave        through the observation field.

Other characteristics and advantages of the invention appear from thefollowing description of an embodiment thereof, given by way ofnon-limiting example and with reference to the accompanying drawing.

In the drawing, FIG. 1 is a diagrammatic view of a shear-wave imagingdevice in an embodiment of the invention.

The imaging device 1 shown in FIG. 1 is for studying the propagation ofelastic shear waves in a viscoelastic medium 2 that diffuses ultrasoundwaves in compression, and that may be constituted, for example:

-   -   by an inert body, in particular for quality control in        industrial applications; or    -   a living body, for example a portion of the body of a patient,        in medical applications.

By way of example, these movements are tracked by means of amicrocomputer 4 (comprising at least an input interface 4 a such as akeyboard, etc., and an output interface such as a screen, etc.) or anyother electronic central unit, serving to send ultrasound compressionwaves into the medium 2 from its outside surface 3, which waves interactwith diffusing particles 5 contained in the medium 1, which particlesare reflective for ultrasound compression waves. The particles 5 may beconstituted by any non-uniformity in the medium 1, and in particular, ina medical application, they may be constituted by particles of collagenpresent in human tissues (in echographic images, such particles formpoints known as “speckle”).

To observe the propagation of the shear wave, an ultrasound probe 6 isused that is disposed against the outside surface 3 of the observedmedium 1. This probe delivers ultrasound compression wave pulses alongan axis X, which pulses are of the type commonly used in echography, forexample having a frequency lying in the range 0.5 MHz to 100 MHz, andpreferably in the range 0.5 MHz to 15 MHz, e.g. being about 4 MHz.

The ultrasound probe 6 is constituted by an array of n ultrasoundtransducers T1, T2, . . . , Ti, Tn, where n is an integer not less than1.

By way of example, the probe 6 may be in the form of a linear stripcapable of comprising, for example, n=128 transducers in alignment alongan axis Y that is perpendicular to the axis X. However, the probe inquestion could equally be in the form of a two-dimensional array oftransducers (plane or otherwise).

The transducers T1, T2, . . . , Tn are controlled independently of oneanother by the microcomputer 4, possibly via a central unit CPU which iscontained for example in an electronics rack 7 connected via a flexiblecable to the probe 6. The transducers T1-Tn can thus emit selectively:

-   -   either an ultrasound compression wave that is “plane” (i.e. a        wave whose wave front is rectilinear in the X,Y plane), or any        other type of unfocused wave illuminating the entire observation        field in the medium 2, for example a wave generated by causing        random sound signals to be emitted by the various transducers        T1-Tn;    -   or else an ultrasound compression wave that is focused on one or        more points of the medium 2.

To observe the propagation of the shear wave in the medium 2, severalsteps are performed in succession:

-   -   a) an excitation step during which the microcomputer 4 causes an        elastic shear wave to be generated in the viscoelastic medium 2        by causing at least one ultrasound wave that is focused in the        viscoelastic medium to be emitted by the probe 6;    -   b) an observation step during which the propagation of the shear        wave is observed simultaneously at a multitude of observation        field points in the viscoelastic medium 2, this observation step        comprising the following substeps:        -   b1) the microcomputer 4 causes the probe 6 to emit into the            viscoelastic medium a succession of unfocused ultrasound            compression wave shocks at a rate of at least 500 shots per            second (the focusing and the timing of the focus ultrasound            wave emitted in step a), and the timing of said unfocused            ultrasound wave are adapted so that at least some of said            unfocused ultrasound waves reach the observation field            during the propagation of the shear wave through the            observation field, for at least some of the unfocused            ultrasound wave emissions;        -   b2) the microcomputer 4 causes the probe 6 to detect and            record in real time sound signals received from the            viscoelastic medium 2, said signals comprising echoes            generated by the unfocused ultrasound compression wave            interacting with the reflecting particles 5 in the            viscoelastic medium, these echoes corresponding (directly or            indirectly) to successive images of the displacement of the            viscoelastic medium;    -   c) and at least one processing step during which:        -   c1) the microcomputer 4 processes the successive sound            signals received from the viscoelastic medium 2 during            substep b2) in order to determine successive propagation            images; and        -   c2) the microcomputer 4 determines at least one movement            parameter for the viscoelastic medium 2 at various points in            the observation field.

The focused ultrasound wave emitted during the excitation step a) may bea monochromatic wave of frequency f lying in the range 0.5 MHz to 15MHz, for example being equal to about 4 MHz, which is emitted for aduration of k/f seconds, where k is an integer lying in the range 50 to5000 (e.g. being about 500) and f is expressed in Hz. Such a wave maypossibly be emitted during a succession of emission periods separated byrest periods, the emission periods following one another at a rate lyingin the range 10 to 1000 emissions per second.

In a variant, the focused ultrasound wave emitted during excitation stepa) is a linear combination (in particular a sum) of two monochromaticsignals of respective frequencies f1 and f2 such that 20 Hz≦|f1−f2|≦1000Hz, thus producing an amplitude modulated wave having a modulationfrequency |f1−f2|.

In addition, the focused ultrasound wave emitted during excitation stepa) may optionally be focused simultaneously or otherwise on a pluralityof points so that the shear wave as generated presents a desired waveshape (for example it is thus possible to generate a shear wave that isplane, or on the contrary a shear wave that is focused) and illuminatesdesired zones in the. medium 2.

During step b1), which may last for example for less than one second, itis possible to emit unfocused ultrasound compression waves at a ratelying in the range 500 to 10,000 shots per second, and preferably in therange 1000 to 5000 shots per second (with this rate being limited by thego-and-return travel time for the compression wave through the medium 2,i.e. by the thickness of the medium 2 in the direction X: it isnecessary for all of the echoes that are generated by the compressionwave to have been received by the probe 6 before a new compression waveis sent).

Each unfocused ultrasound compression wave propagates through the medium2 at a propagation speed that is much higher than that of shear waves(e.g. about 1500 meters per second (m/s) in the human body), andinteracts with the reflecting particles 5, thereby generating echoes orother analogous disturbances in the signal that are known in themselvesunder the name “speckle noise” in the field of echography.

The speckle noise is picked up by the transducers T1, . . . , Tn duringsubstep b2), after each shot of an unfocused ultrasound compressionwave. The signal sij(t) as picked up in this way by each transducer Tiafter shot No. j is initially sampled at high frequency (e.g. 30 MHz to100 MHz) and is digitized in real time (e.g. on 12 bits) by a samplerforming part of the rack 7 and connected to said transducer, thesamplers being referenced respectively E1, E2, . . . , En.

The signal sij(t) as sampled and digitized in this way is then stored,likewise in real time, in a memory Mi belonging to the rack 7 andspecific to the transducer Ti.

By way of example, each memory Mi presents a capacity of about 128megabytes (MB), and contains all of the signals sij(t) received insuccession for shots j=1 to p.

In deferred time, after all of the signals sij(t) corresponding to thesame propagation of a shear wave have been stored, the central unit CPUcauses these signals to be reprocessed by a summing circuit S belongingto the rack 7 (or else it performs this treatment itself, or indeed thetreatment may be performed in the microcomputer 4), using a conventionalpath-forming step corresponding to substep c1).

This generates signals Sj(x,y) each corresponding to the image of theobservation field after shot No. j (when the unfocused ultrasound waveis a plane wave).

For example, it is possible to determine a signal Sj(t) by the followingformula:${{Sj}(t)} = {\sum\limits_{i = 1}^{n}{{\alpha_{i}\left( {x,y} \right)} \cdot {{sij}\left\lbrack {{t\left( {x,y} \right)} + {{d_{i}\left( {x,y} \right)}/V}} \right\rbrack}}}$where:

-   -   sij is the raw signal perceived by the transducer No. i after        ultrasound compression wave shot No. j;    -   t(x,y) is the time taken by the ultrasound compression wave to        reach the point of the observation field having coordinates        (x,y), with t=0 at the beginning of shot No. j;    -   d_(i)(x,y) is the distance between the point of the observation        field having coordinates (x,y) and transducer No. i, or an        approximation to said distance;    -   V is the mean propagation speed of ultrasound compression waves        in the viscoelastic medium under observation; and    -   α_(i)(x,y) is a weighting coefficient taking account of        apodization relationships (in practice, in numerous cases, it is        possible to assume that α_(i)(x,y)=1).

The above formula applies mutatis mutandis when the observation field isthree-dimensional (with a two-dimensional array of transducers), withspace coordinates (x,y) being replaced by (x,y,z).

After the optional path-forming step, the central unit CPU stores in acentral memory M forming part of the rack 7, the image signals Sj(x,y),or Sj(x), or Sj(x,y,z), each corresponding to shot No. j. These signalsmay also be stored in the microcomputer 4 if it is the microcomputerthat performs the image processing itself.

These images are then processed in deferred time in substep c2) bycorrelation and advantageously by cross-correlation either in pairs, orpreferably with a reference image which may be:

-   -   either a displacement image determined previously as explained        above and used as a reference image for subsequent displacement        images (or for a limited number of subsequent displacement        images), e.g. 30 displacement images);    -   or else determined during a preliminary initial observation step        a0), like the above-mentioned successive displacement images, by        causing one or more unfocused ultrasound waves to be emitted        by-the probe 6 before excitation step a) which generates the        shear wave (when a plurality of unfocused ultrasound compression        waves are emitted in this way prior to the excitation stage,        echoes generated by each unfocused compressed ultrasound wave        are recorded interacting with the reflecting particles in the        viscoelastic medium, these echoes corresponding to a plurality        of successive preliminary images of the viscoelastic medium, and        said initial image of the viscoelastic medium is determined by        combining said successive preliminary images, and in particular        by averaging the pixel values of said preliminary images).

The above-mentioned cross-correlation can be performed, for example, ina specialized digital signal processor (DSP) electronic circuitbelonging to the rack 7, or it may be programmed in the central unit CPUor in the microcomputer 4.

During this cross-correlation process, a cross-correlation function<Sj(x,y),Sj+1(x,y)> is maximized in order to determine the displacementto which each particle 5 giving rise to an ultrasound echo has beensubjected.

Examples of such cross-correlation calculations are given in the stateof the art, in particular by O'Donnell et al. in “Internal displacementand strain imaging using speckle tracking”, IEEE transactions onultrasound, ferroelectrics, and frequency control, Vol. 41, No. 3, May1994, pp. 314-325, and by Ophir et al. in “Elastography: a quantitativemethod for imaging the elasticity of biological tissues”, UltrasoundImag., Vol. 13, pp. 111-134, 1991.

This produces a set of displacement vectors {overscore (u)}({overscore(r)},{overscore (t)}) generated by the shear waves in each position{overscore (r)} of the medium 2 under the effect of the shear wave(these displacement vectors may optionally be reduced to a singlecomponent in the example described herein).

This set of displacement vectors is stored in the memory M or in themicrocomputer 4 and can be displayed, for example, in particular bymeans of the screen 4 a of the computer, in the form of a slow motionpicture in which the values of the displacements are illustrated by anoptical parameter such as a gray level or a color level.

The propagation differences of the shear wave between zones havingdifferent characteristics in the medium 2 can thus be seen clearly, forexample the zones may comprise healthy tissue and cancerous tissue in amedical application.

The motion picture of shear wave propagation can also be superposed on aconventional echographic image, which can also be generated by theapparatus 1 described above.

Furthermore, it is also possible to calculate not the displacements ofeach of the points in the observed medium 2, but the deformations of themedium 2, i.e. vectors whose components are the derivatives of thedisplacement vectors respectively relative to the space variables (X andY coordinates in the example described). These deformation vectors canbe used like the displacement vectors for clearly viewing thepropagation of the shear wave in the form of a motion picture, and theyalso present the advantage of eliminating displacements of the probe 6relative to the medium 2 under observation.

From the displacement or deformation fields, the microcomputer 4 canadvantageously then proceed with a map-making step d) during which, onthe basis of the way in which the movement parameter (displacement ordeformation) varies over time in the field of observation X, Y (or X, Y,Z with a two-dimensional array of transducers), it calculates at leastone propagation parameter of the shear wave, either at certain points inthe observation field as selected by the user acting on themicrocomputer 4, or else throughout the observation field.

The propagation parameter of the shear wave that is calculated duringthe map-making step is selected, for example, from amongst: thepropagation speed c_(s) of shear waves, the shear modulus μ, or Young'smodulus E=3μ, the attenuation α of the shear waves, the shear elasticityμ1, the shear viscosity μ2, or the mechanical relaxation time τ_(s) ofthe tissues.

For example, it is possible at various points in the observation fieldto calculate:

-   -   the value of the propagation speed c_(s) of the shear wave, thus        giving information about the hardness of the tissues;    -   the value of the mechanical relaxation time τ_(s) of the        tissues, which is characteristic of the local viscosity of the        medium.

To do this, the following propagation equation (1) is used, with thedisplacement {overscore (u)}({overscore (r)},{overscore (t)}) generatedby the shear waves at each position {overscore (r)} of the mediumsatisfying this equation: $\begin{matrix}{{\rho\frac{{\partial^{2}p}{\overset{\_}{u}\left( {\overset{\_}{r},t} \right)}}{\partial t^{2}}} = {{c_{s}^{2}\left( {1 + {\tau_{s}\frac{\partial}{\partial t}}} \right)} \cdot {{\overset{\_}{\nabla}}^{2}{\overset{\_}{u}\left( {\overset{\_}{r},t} \right)}}}} & (1)\end{matrix}$where ρ is the density of the tissues, τ_(s) is the mechanicalrelaxation time of the tissues, and c_(s) is the propagation speed ofthe shear wave which is directly related to Young's modulus E of thetissue by the following relationship: $\begin{matrix}{c_{s} = \sqrt{\frac{E}{3\rho}}} & (2)\end{matrix}$

In the Fourier domain, the above wave equation (1) can be written asfollows:ω² ρU({overscore (r)},ω)=c _(s) ²(1+jωτ).ΔU({overscore (r)},ω)   (3)where U({overscore (r)},ω) is the Fourier transform of the displacementfield {overscore (u)}({overscore (r)},t) measured at each point, andΔU({overscore (r)},ω) is the Fourier transform of the spatial Laplacianof the field {overscore (u)}({overscore (r)},t). Given that ωτ_(s)<<1,it is possible to use a simplified expression: $\begin{matrix}{c_{s}^{2} = {\omega^{2}\rho\quad\frac{U\left( {\overset{\_}{r},\omega} \right)}{\Delta\quad{U\left( {\overset{\_}{r},\omega} \right)}}}} & (4) \\{\tau_{s} = {\frac{1}{\omega}{\tan\left( {\Psi\left( \frac{U\left( {\overset{\_}{r},\omega} \right)}{\Delta\quad{U\left( {\overset{\_}{r},\omega} \right)}} \right)} \right)}}} & (5)\end{matrix}$where ψ(x) is the phase of the complex variable x. The functionsU({overscore (r)},ω) and ΔU({overscore (r)},ω) are known for each pointof the echographic image, so it is possible to measure Young's modulusand the mechanical relaxation time of the tissue at each point in thespace, thereby drawing up a map of those two parameters.

Since equations (4) and (5) are true at each frequency, the calculationof c_(s) and τ_(s) can advantageously be averaged over the entire bandof frequencies carried by the shear wave, thereby greatly improving thequality of the mapping that is performed. For this purpose, it ispossible to use the following formulae: $\begin{matrix}{c_{s}^{2} = {\frac{1}{\omega_{1} - \omega_{0}}{\int_{\omega}^{\omega_{1}}{\omega^{2}\rho\frac{U\left( {\overset{\_}{r},\omega} \right)}{\Delta\quad{U\left( {\overset{\_}{r},\omega} \right)}}{\mathbb{d}\omega}}}}} & (6) \\{\tau_{s} = {\frac{1}{\omega_{1} - \omega_{0}}{\int_{\omega}^{\omega_{1}}{\frac{1}{\omega}{\tan\left( {\Psi\left( \frac{U\left( {\overset{\_}{r},\omega} \right)}{\Delta\quad{U\left( {\overset{\_}{r},\omega} \right)}} \right)} \right)}{\mathbb{d}\omega}}}}} & (7)\end{matrix}$where ω₀ and ω₁ are the minim and maximum frequencies carried by theshear wave.

The method of calculation would be the same when using not displacementsbut deformations in the observed medium 2.

Furthermore, it is advantageous to establish a succession of severalmaps of the desired propagation parameters, e.g. c_(s) and τ_(s), bygenerating successive different shear waves, e.g. obtained by emittingultrasound compression waves focused successively on a plurality ofpoints or having different wave shapes. It is then possible to combinethe various maps that are obtained in this way, e.g. by averaging them,so as to obtain a combination map that is richer and more accurate.

1. An imaging method using shear waves for observing a diffusingviscoelastic medium containing particles that reflect ultrasoundcompression waves, said method comprising: a) an excitation step duringwhich an elastic shear wave is generated in the viscoelastic medium; b)an observation step during which the propagation of the shear wave isobserved simultaneously at a multitude of points in an observation fieldin the viscoelastic medium, this observation step comprising thefollowing substeps: b1) causing an array of transducers that arecontrolled independently of one another to emit into the viscoelasticmedium a succession of unfocused ultrasound compression wave shots at arate of at least 500 shots per second; and b2) causing sound signalsreceived from the viscoelastic medium to be detected and recorded inreal time, said signals comprising the echoes generated by the unfocusedultrasound compression wave interacting with the reflecting particles insaid viscoelastic medium; and c) at least one processing step duringwhich: c1) the sound signals received successively from the viscoelasticmedium during substep b2) are processed in order to determine successivepropagation images of the shear wave; and c2) at least one movementparameter of the viscoelastic medium is determined at different pointsof the observation field; the method being characterized in that duringexcitation step a) the elastic shear wave is caused to be generated bycausing at least one focused ultrasound wave to be emitted into theviscoelastic medium by said array of transducers, the focusing and thetiming of said focused ultrasound wave, and the timing of said unfocusedultrasound wave being adapted so that at least some of said unfocusedultrasound waves penetrate into the observation field while the shearwave is propagating in the observation field, for at least some of theunfocused ultrasound wave emissions.
 2. A method according to claim 1,in which during substep b2), in order to determine said movementparameter, a plurality of successive propagation images are comparedwith a common reference image of the viscoelastic medium, the referenceimage being determined by firing at least one unfocused ultrasoundcompression wave into said viscoelastic medium and then detecting andrecording echoes generated by said unfocused ultrasound compression waveon interacting with the reflecting particles in the viscoelastic medium.3. A method according to claim 2, in which step a) is preceded by aninitial observation step a0) during which at least one unfocusedultrasound compression wave is fired and then echoes generated by saidunfocused ultrasound compression wave interacting with the reflectingparticles in the viscoelastic medium are detected and recorded, saidechoes corresponding to an initial image of the viscoelastic medium, andduring substep b2), said initial image constitutes said reference imagefor processing at least some of the successive displacement images.
 4. Amethod according to claim 3, in which, during initial observation stepa0), a plurality of unfocused ultrasound compression waves are fired insuccession and then echoes generated by each unfocused ultrasoundcompression wave interacting with the reflecting particles of theviscoelastic medium are detected and recorded, said echoes correspondingto a plurality of successive images of the viscoelastic medium, and saidinitial image of the viscoelastic medium is determined by combining saidsuccessive images.
 5. A method according to claim 1, in which saidmovement parameter is a displacement of the viscoelastic medium.
 6. Amethod according to claim 1, in which the focused ultrasound waveemitted during excitation step a) presents a frequency f lying in therange 0.5 MHz to 15 MHz, and is emitted for a duration of k/f seconds,where k is an integer lying in the range 50 to 5000 and f is expressedin Hz.
 7. A method according to claim 1, in which the focused ultrasoundwave emitted during excitation step a) presents a frequency lying in therange 0.5 MHz to 15 MHz and is emitted during a succession of emissionperiods separated by rest periods, the emission periods following oneanother at a rate lying in the range 10 to 1000 emissions per second. 8.A method according to claim 1, in which the focused ultrasound waveemitted during excitation step a) is a linear combination (in particulara sum) of two monochromatic signals having respective frequencies f1 andf2 such that 20 Hz≦|f1−f2|≦1000 Hz.
 9. A method according to claim 1, inwhich the focused ultrasound wave emitted during excitation step a) isfocused simultaneously on a plurality of points.
 10. A method accordingto claim 1, in which image processing step c) is followed by a mappingstep d) during which, on the basis of variation in the movementparameter over time, at least one shear wave propagation parameter iscalculated at at least some points of the observation field in order todetermine a map of said propagation parameter in the observation field.11. A method according to any preceding claim 1, in which the shear wavepropagation parameter which is calculated during mapping step d) isselected from shear wave speed, shear modulus, Young's modulus, shearwave attenuation, shear elasticity, shear viscosity, and mechanicalrelaxation time.
 12. A method according to claim 11, in which steps a)to d) are repeated successively while emitting different focusedultrasound waves during successive excitation step a), and thencombining the maps obtained during the successive mapping step d) inorder to calculate a combination map of the observation field. 13.Imaging apparatus for implementing a method according to claim 1 usingshear waves to observe a diffusing viscoelastic medium containingparticles that reflect ultrasound compression waves, the apparatuscomprising an array of transducers that are controlled independently ofone another by at least one electronic central unit adapted: to cause atleast one elastic shear wave to be generated in the viscoelastic medium;to observe the propagation of the shear wave simultaneously at amultitude of points in an observation field in the viscoelastic mediumby causing said array of transducers to emit into the viscoelasticmedium a succession of unfocused ultrasound compression wave shots at arate of at least 500 shots per second, then causing said array oftransducers to detect in real time and to record in real time soundsignals received from the viscoelastic medium, the sound signalscomprising the echoes generated by the unfocused ultrasound compressionwave interacting with the reflecting particles of said viscoelasticmedium; and processing the successive sound signals received from theviscoelastic medium to determine successive propagation images of theshear wave, and then determining at least one movement parameter of theviscoelastic medium at different points of the observation field; theapparatus being characterized in that the electronic central unit isadapted to cause the elastic shear wave to be generated by causing atleast one focused ultrasound wave to be emitted into the viscoelasticmedium by said array of transducers, the focusing and the timing of saidfocused ultrasound wave, and the timing of said unfocused ultrasoundwave being adapted so that said unfocused ultrasound waves reach theobservation field during the propagation of the shear wave through theobservation field.